Power semiconductor transistors, such as power metal-oxide-semiconductor field-effect transistors (MOSFETs) and insulated gate bipolar transistors (IGBTs), are typically fabricated with silicon (Si) semiconductor material. Recently, silicon carbide (SiC) power devices have been considered due to superior properties. Gallium nitride (GaN) devices have also emerged as attractive candidates to carry large currents, support high voltages, and provide very low on-resistance and fast switching times.
An ideal power switch has zero conduction and switching loss. Conduction loss limits the maximum operating voltages of current unipolar devices to around 600 V, which otherwise have very low switching loss due to the absence of stored charge. Bipolar devices such as IGBTs can overcome this problem, but generally suffer from high switching loss.
Typical power semiconductor transistors can have both lateral and vertical topologies. A vertical structure for a power device is often better suited for handling relatively high power (e.g., on the order of several kWs). The breakdown voltage of such devices is not limited by edge effects, but rather by material properties because the electric field is buried in the bulk of the material. GaN devices use a modified version of a double-defused metal-oxide-semiconductor field-effect transistor (MOSFET) (sometimes referred to as a DMOS) structure used in Si, that is sometimes referred to as a current aperture vertical electron transistor (CAVET). A CAVET generally has a source formed by a two-dimensional electron gas in the GaN at the AlGaN/GaN interface. A drain includes a drift layer of lightly doped n-type GaN with a heavily doped n-layer to form an ohmic contact for the drain. A current blocking layer (CBL) which either provides a potential barrier, or is insulating, separates the source from the drain. Thus the CBL can either be formed by p-type GaN (grown or implanted), by using an insulating layer, or a higher bandgap material like AlN. An aperture in the current blocking layer which is formed using conductive material can insure that the current flow from the source to drain is vertical.
Power electronic switches fabricated using GaN are generally limited by the material properties such as, for example, a bandgap of 3.2 eV at 300 degrees Kelvin (K) which corresponds to a breakdown field of ˜5 megavolts per centimeter (MV/cm). The thermal properties of GaN can limit the operational ability of these devices, which is sometimes addressed in high electron mobility transistors (HEMTs) by using diamond as a heat sink.
Considering such limitations of approaches using GaN and similar semiconductor materials, new systems and methods for a vertical transistor that has a larger bandgap and improved thermal properties would be desirable.